In a television system, tuners are generally used to extract signals within a desired channel having a channel width of 6 MHz out of a broad television spectrum having a range of approximately 54˜860 MHz (cable systems) or 54˜806 MHz (broadcast systems in the U.S.). Tuners generally also convert the extracted signals into a standard intermediate frequency (IF) so that subsequent devices in the television system may process the signals at such standard IF. In general applications in the U.S., the standard IF is 45.75 MHz.
Conventional tuners generally convert the signals within the desired channel to the standard IF by mixing the signals with a local oscillation (LO) signal using a mixer. A mixer mixing an input signal with an LO signal would generate two output signals, one at a sum (sum frequency) of the frequencies of the input signal and the LO signal and one at a difference (difference frequency) between the frequencies of the input signal and the LO signal. Typically, signals at the difference frequency will be selected, while signals at the sum frequency will be filtered out. Therefore, if the frequency of the LO signal is apart from a frequency of the desired channel by an amount equal to the standard IF, a mixer mixing the LO signal with the desired channel would convert the signals in the desired channel into the standard IF. Thus, for example, if the desired channel is transmitted at 100 MHz, the LO signal may have a frequency of 145.75 MHz or 55.25 MHz. However, a mixer cannot differentiate a case where the frequency of the LO is higher than that of the desired channel from another case where the frequency of the LO is lower than that of the desired channel. In other words, in converting signals of the desired channel to the standard IF, a first signal at a frequency above and spaced apart from that of the LO signal by an amount equal to the standard IF would be converted to the standard IF as well as a second signal at a frequency below and spaced apart from that of the LO signal by an amount equal to the standard IF. For example, if the desired channel is at 100 MHz and the LO signal has a frequency of 145.75 MHz, a signal at 191.5 MHz would also be converted into 45.75 MHz and interfere with the signals in the desired channel. Such an interfering signal is generally referred to as an image signal and must be suppressed.
Conventional tuners are generally of one of two types, namely, a single-conversion tuner or a dual-conversion tuner. A single-conversion tuner utilizes a tracking filter to suppress image signals, while a dual-conversion tuner converts the television signals to a first intermediate frequency and then utilizes a filter with fixed parameters to suppress image signals. An example of each type is discussed below.
FIGS. 1A-1I illustrate aspects of a conventional single-conversion tuner 100, wherein FIG. 1A shows the structure of tuner 100. FIGS. 1B-1E illustrate frequency spectrums of the television signals at different stages of tuner 100 for a first example in which a low channel is to be selected. FIGS. 1F-1I illustrate frequency spectrums of the television signals at different stages of tuner 100 for a second example in which a high channel is to be selected.
Single-conversion tuner 100 receives television signals having a frequency spectrum ranging from a low frequency fL to a high frequency fH and selects a desired channel out of the television signals. As shown in FIG. 1A, tuner 100 includes a low-noise amplifier (LNA) 102 for receiving and amplifying the television signals, a tracking filter 104 for filtering out image signals of the desired channel, a local oscillator 106 providing an LO signal having a frequency fLO higher than the desired channel by an amount equal to the standard IF, a mixer 108 for mixing the television signals with the LO signal, and a channel select filter (CSF) 110 for selecting the desired channel of signals.
FIG. 1B shows a frequency spectrum of the television signals after being received and amplified by LNA 102. In the first example, a low channel within the frequency spectrum of the television signals, having a center frequency of fc1 towards a lower end of the frequency spectrum, is to be selected. The frequency of the LO signal, fLO, is indicated in FIG. 1B to be higher than fc1 by an amount equal to the standard IF, or 45.75 MHz, which is labeled as “IF” in FIGS. 1B-1I. An image signal band (or image signal) is also shown in FIG. 1B to be above and spaced apart from fLO by an amount equal to IF. In FIG. 1B, the image signal band has a center frequency of fi1, and fi1−fLO=fLO−fc1=IF. Also shown in FIG. 1B is a filter response of tracking filter 104, labeled as curve HT. The image signal band falls outside a pass band of tracking filter 104. Therefore, the image signal band may be suppressed by passing the television signals through tracking filter 104.
FIG. 1C shows a frequency spectrum of the television signals after being filtered by tracking filter 104. As shown in FIG. 1C, the portion of the spectrum within the pass band of tracking filter 104 is preserved, while the portion outside the pass band of tracking filter 104, including the image signal band, is substantially suppressed.
After being filtered by tracking filter 104, the television signals are mixed by mixer 108 with the LO signal provided by local oscillator 106. As a result, the television signals are converted to difference frequencies between the LO signal and the television signals, as shown in FIG. 1D. Also as shown in FIG. 1D, both remnants of the image signal and the signals within the desired channel are converted to the standard IF, and the portion of the spectrum denoted as i1 corresponds to the remnants of the image signal. FIG. 1D further shows a filter response of CSF 110, labeled as curve HCSF. The pass band of CSF 110 has a bandwidth of approximately 6 MHz and is centered at approximately 45.75 MHz. After the signals pass through CSF 110, the signals within the desired channel are selected, with the remnants of the image signal thereof substantially suppressed, as shown in FIG. 1E.
In the second example, a high channel within the frequency spectrum of the received television signals is to be selected. A filtering response of tracking filter 104 is accordingly adjusted such that the desired high channel falls within the pass band of tracking filter 104 while the image signal of the desired high channel is rejected as falling outside that pass band. FIG. 1F shows the frequency spectrum of the television signals after being received and amplified by LNA 102. In the amplified signals, a high channel having a center frequency of fc2 lies within and towards a higher end of the frequency spectrum of the television spectrum. fLO is higher than fc2 by an amount equal to the standard IF, or 45.75 MHz. An image signal band, or an image signal, has a center frequency fi2 that is higher than fLO by 45.75 MHz. A filter response of tracking filter 104, labeled as curve HT, is also shown in FIG. 1F. The image signal band falls outside a pass band of tracking filter 104, while the desired high channel falls within that pass band. FIGS. 1G-1I respectively illustrate the frequency spectrums of the signals as they pass through tracking filter 104, mixer 108, and CSF 110. These spectrums should be understood by one skilled in the art and are therefore not explained in detail herein.
As discussed above, when selecting different channels (a low channel and a high channel in the above examples), a filtering response of tracking filter 104 must be adjusted accordingly. Tracking filters as such are generally powered by a 30V source and are difficult to integrate.
To obviate the problems associated with a tracking filter, there have been provided dual-conversion tuners for use in television systems, an example of which is disclosed in U.S. Pat. No. 6,177,964 to Birleson et al., and is discussed below with reference to FIGS. 2A-2E.
FIG. 2A shows a structure of a dual-conversion tuner 200 according to Birleson et al. Tuner 200 includes an LNA 202 for receiving and amplifying television signals, a first mixer 204 for mixing the amplified television signals with a first LO signal having a frequency fLO1, a band pass filter (BPF) 206 for filtering the signal output of mixer 204, a second mixer 208 for mixing the output of BPF 206 with a second LO signal having a frequency fLO2, and a CSF 210 for selecting a desired channel. FIG. 2B shows the frequency spectrum of the received television signals, including the desired channel, which has a center frequency fc. fLO1 is higher than fc by an amount equal to a first intermediate frequency IF1. IF1 may be selected such that fLO1 is outside the frequency spectrum of the television signals, and preferably far away from the frequency spectrum. As a result, a corresponding first image signal fi1 of the desired channel with respect to fLO1 would also be far away from the frequency spectrum, and may be easily suppressed by a filter (not shown) prior to LNA 202. FIG. 2B shows the remnants of the first image signal fi1 after being suppressed by the filter prior to LNA 202. FIG. 2C shows the frequency spectrum of the signal output of first mixer 204, wherein a portion of the spectrum corresponding to the desired channel lies at a frequency equal to IF1 and the portion of the spectrum denoted as i1 corresponds to the remnants of the first image signal. A filtering response of BPF 206 is illustrated in FIG. 2C as curve HBPF. FIG. 2D shows the frequency spectrum of the television signals after being filtered by BPF 206. The frequency of the second LO signal, fLO2, is lower than IF1 by an amount equal to the standard intermediate frequency, denoted as IF2. A corresponding second image signal with respect to fLO2 is at frequency fi2=fLO2−IF2, as indicated in FIG. 2D. Then, after second mixer 208 and CSF 210, the signals of the desired channel are filtered out, as shown in FIG. 2E, where the portion of the spectrum denoted as i1+i2 corresponds to the remnants of the first image signal and the second image signal.
Tuner 200 as shown in FIG. 2A does not require a tracking filter, and therefore does not require a 30V power supply. However, tuner 200 still requires two filters, BPF 206 and CSF 210, which may not be easily integrated with the rest of the circuit.